Make sure empty HLOD actors are irrelevant during cook

HLOD actors may be empty and have no meshes generated for them in which case they should not be included in the cooked game.

• Place some HLOD relevant actors in a WP map, save it
• Using the WP builder commandlet, run only the -SetupHLODs step
• Cook

Result: During execution, you will get spammed with "has no root component in AActor::IsNetRelevantFor()"

Expected: No errors